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Wednesday, 4 July 2012

Worlds on the edge of knowledge: Salacia and Actaea

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You'll notice a lot of words like 'probably', and 'may' in this post -  this is because the worlds I'm posting about today really are on the edge on knowledge - what we know about them is a best, educated, guess. So take this as a disclaimer: What we know about these places might change tomorrow.

There are a myriad of small, lonely worlds, out beyond the orbit of Neptune -  they make up the massive, sparse, Kuiper belt [1].

Most of these appear only as points of light even to the most powerful telescopes: Pluto is amongst the largest, and this is the best shot Hubble [2] can get of it:

Image above: The Hubble Space Telescopes best view of Pluto. It's a technical marvel. But it's just a bit on the fuzzy side... Image courtesy of NASA.

See? If I take off my glasses and look at a ball of orange wool I get more or less the same view.....

...Yet even a tiny point of light can carry a lot of information from the edge of the interstellar gulf. Decoding that information takes patience, a few well chosen assumptions, a deep understanding of the physics of these objects, and of the instruments used to observe them.

Image above: A map of the outer solar system, showing the Kuiper belt relative to the giant planets. The inner edge of the belt begins just past the orbit of Neptune, and extends twenty times the distance from the Earth to the Sun. Image courtesy of NASA

John Stansberrys research group [3] does just this kind of deep space detective work. Using the Hubble Space telescope and Spitzer (infra red) Space Telescope [4], they've been able to sketch out an impression of a pair of little worlds in the belt: Salacia, and its moon Actaea.

And, unsurprisingly in my opinion, the pair have surprised us:

 Image above: The biggest little worlds in the Kuiper belt: Salacia is about the same size as Sedna. But  Salacia is a sexier name. Does thinking that make me odd? Image courtesy of NASA.

The first surprise is that these worlds aren't so little: Salacia is between eight hundred and one thousand kilometres wide, making it bigger than Vesta, and possibly a little bigger than Ceres. It's not just an oversized comet, like many Kuiper belt objects, but a true dwarf planet. Actaea looms massively  in the Salacian sky - it's around three hundred kilometres across, and orbits only five thousand six hundred kilometres from Salacia. That makes them  the closest orbiting pair found so far in the belt.
And then the sketch of this place gets odder: Salacia is the blackest thing found in the belt so far, reflecting only four percent of the weak sunlight that hits it - which probably means a surface covered with carbon based chemicals [5], perhaps including precursors of life [6]. That idea is supported by it's infra red spectrum [7], which shows less than five percent water ice. Most objects that big out there are much brighter, probably because they've had some kind of internal heating in their past - so it follows that Salacia has seen almost none.

Salacia is big but light weight, only slightly denser than water ice. But it should contain some rock, which is much, much, denser than ice. So there must be voids underground, to explain the low density. There's no way of telling if these are huge caverns, or lots of tiny cracks, but we can be sure of this: They won't have seen light since the solar system formed.

So, we can put ourselves into a sketch of Salacia. It's spectacular in its way, but no holiday spot, not even for a terminal Lovecraft addict: This is a world of black stained ice, under the dim rays of a Sun forty five times more distant than from Earth. The horizon is unnaturally close, sharp, and visible mainly because it blocks out the billions of untwinkling stars. A black moon hangs massively in the sky. The ground is probably rotten with warrens of four point five billion year old caverns. Those cavern walls may hold the preserved chemical make up of the pre-solar nebula, untouched by warmth, or even the feeble sunlight of the Kuiper belt...

.... I mean, if you stepped off the plane and found that waiting for you, you'd say: " This doesn't look like a good place to get a tan," or " how am I supposed to ski on that?".

Although, first, you'd probably ask: "Why am I on the furthest edge of the solar system, when the brochure said Tenrife?" and "who the hell is my travel agent?". Assuming you plane had a stock of space suits, so you could breathe.

Which isn't likely.

Oh, and despite the terrifying distance to the desolate part of space it resides in, Actaea has it's own facebook page [8]. I can't decide if that's brilliant or slightly scary, but it's worth mentioning, because I couldn't make this stuff up!

Finally, to give the sketch some context, here's a tour of the ice giants and beyond, Salacias neighbourhood:

Video above: A very impressive CGI trip to the edge of the known solar system. And it's an 'amateur' effort! Credits given at the end.

Salacia may never be much more to us than a spot of light - it'll take nearly nine years just to get one probe to Pluto [9], and Pluto is famous! But as telescope technology improves, and methods of analysing light get more sophisticated, we will learn more about this black-ice world. And it seems that Salacia and its moon may have secrets to tell about where our solar system came from....

For more, check out Emily Lakdawallas blog [10], and the abstract [11] of John Stansberrys paper.

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